The invention relates to an apparatus for positioning movable aerodynamic control surfaces on the wings of an aircraft in response to operation of both a pilot-controlled roll input (produced by turning the control wheel) and a pilot-controlled speed-brake input (produced by operating a lever located near the control wheel). While the invention is described herein in relation to its use for controlling the positions of spoilers (i.e., moveable aerodynamic control surfaces located on the upper surfaces of the wings of an aircraft), it will be apparent that the invention may also be used for controlling the positions of other types of aerodynamic control surfaces.
Devices for positioning wing mounted aerodynamic control surfaces in response to two or more pilot control inputs are general known and are sometimes referred to as "mechanical mixers". The roll and speed-brake inputs are typically applied to the mixer via control cables that are movable in response to the control wheel and the speed-brake lever, respectively. Controlled movements of the cables selectively displace a series of interconnected shafts, cranks cams and levers that comprise the mixer and that mechanically combine the inputs and produce a pair of related outputs, one for each wing. The outputs are in turn transmitted via control cables to hydraulic servomechanisms which force the spoilers to positions determined by the combined inputs.
Proper positioning of the spoilers must take into account the following aerodynamic factors. When the spoilers are raised by equal increments in response to the speed-brake, the drag on both wings is increased equally thereby providing control over the air speed without producing roll of the aircraft. When the spoilers are deployed by unequal increments in response to the lateral input derived from the control wheel, the spoilers act like ailerons to produce roll of the aircraft. Such unequal deployment occurs substantially in concert with movement of the ailerons and augments the roll response obtained by the ailerons. In some flight maneuvers, the control wheel and speed-brake are used separately and in such case the spoilers are responsive to only one of these controls at any given time. On the other hand, many flight conditions require concurrent use of both these controls so that the pilot can both control the air speed and perform roll manuevers. Known mechanical mixers are in general capable of combining these controls to position the spoilers in a manner that enables such concurrent use. However, with such known devices the pilot must carefully compensate for certain variations in the aircraft's aerodynamic response to one control depending on the setting of the other control.
Specifically, these variations which the pilot must compensate for can be best visualized by considering the following flight conditions. The first such condition is with the aircraft in level flight and the speed-brake lever set to zero so that the spoilers are fully retracted (i.e. in the down position flush with the upper surfaces to the wings). To bank the aircraft, the pilot turns the control wheel (either clock-wise or counter-clockwise from a zero degree roll position), causing the ailerons and spoilers on one wing to be raised. On the other wing the ailerons are lowered, but because the spoilers on such other wing are already fully retracted they cannot be further lowered and thus they merely remain in the down position. The wing having the raised spoilers (and raised ailerons) will incur increased drag and consequent loss in lift, thereby causing it to drop. Conversely, the lift on the opposite wing is increased by the lowered aileron (the spoiler in this case being retracted and thus not affecting lift). The aircraft now goes into a roll at a rate that is a function of the degree of deployment of the spoilers and ailerons. It is noted that the diminished lift of the first mentioned wing is due to the combined effects of both the raised ailerons and the raised spoilers whereas the enhanced lift of the latter wing is due solely to the lowered ailerons. A corresponding aerodynamic response occurs when the speed-brake is at maximum and the control wheel is turned to effect a lowering of both the spoilers and ailerons on one wing. Which together enhance the lift of that wing, while the spoilers on the other wing are at maximum deployment and cannot be further elevated and thus remain fixed so that the diminished lift on such other wing is due solely to the ailerons.
Now in comparison, consider the aircraft, again in level flight, but with the speed-brake input at an intermediate setting such that the spoilers on both wings are partially raised. As such, the spoilers on both wings are capable of either being raised or lowered in response to the control wheel. When the control wheel is turned in one or the other direction, a differential displacement of the spoilers occurs, in concert with the differential displacement of the ailerons, raising them on one wing and lowering them on the other. Because of the differential displacement of the spoilers, the roll response of the aircraft is significantly accentuated relative to the above-described response when the spoilers on only one wing are displaced. Thus, the same increment of control wheel rotation will produce a much sharper roll rate than in the above-described case. Because the pilot must compensate for the change in response that exists between the unilateral and differential displacement of the spoilers, depending on the setting of the speed-brake, control of the aircraft is not as precise as it could be if the roll response of the aircraft were the same or substantially the same for all settings of speed-brake.
To a certain extent the foregoing problem has been alleviated through the use of a variable-length, lever mechanism called a ratio-changer, that coacts with a mechanical mixer to vary the response of the mixer to the roll input as the speed-brake input changes. The ratio-changer thereby attenuates the responsiveness of the output of the mixer to the roll input to roughly compensat for the increased roll sensitivity that occurs when the spoilers are at an intermediate deployment and are capable of being differentially displaced as described above. Because of inherent limitations in the design of the existing ratio-changer and in the manner in which it cooperates with the mixer, such a device is capable of only partially compensating for the change in roll response that accompanies different speed-brake settings.
An example of an existing mixer of the type suitable for being equipped with a ratio-changer is found in the spoiler control system used on the Model 727 aircraft manufactured by The Boeing Company of Seattle, Washington. The mixer portion of such system is disclosed In U.S. Pat. No. 3,166,272, issued to M. S. Liddell et al. on Jan. 19, 1965. As illustrated in the mentioned patent the mixing function is peformed by the combination of a two-dimensional cam having a pair of followers cooperating therewith, and a series of follower arms and interconnecting linkages that cooperate to mix the roll and speed-brake inputs to produce two related outputs that control the position of the spoilers on the opposed wings. The aforementioned patent does not however disclose the above-described ratio-changer.
Accordingly, one object of the present invention is to provide an apparatus for disposing the spoilers of an aircraft in predetermined positions of deployment in response to any pilot-selected combination of roll and speed-brake inputs. A related and particular object of the invention is to provide such an apparatus capable of combining such inputs so that the roll response of the aircraft to the pilot-operated control wheel is substantially independent of the speed-brake input.